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Design Note – DN05004/D

Positive Input to Negative Output Conversion using a Buck Converter

Device Application Input Voltage Output Voltage Topology I/O Isolation

NCP3170

NCP3020 Power supplies Positive Negative Buck None

Input Voltage +5, 8, 12 Vdc Output Voltage -3.3, -5 Vdc Nominal Current 0.5 A or 8 A Nominal Efficiency 80 - 90%

Introduction

The buck-boost is usually the topology of choice for voltage inverting applications. There are plenty of components to choose from and most of them even come with integrated transistors or mosfets. They can work in hysteretic mode or they can switch at a fixed frequency. Very often when more power is required, an external transistor is used and while the circuit can deliver the needed power, the efficiency is very poor.

This Design Note describes an alternate solution that is very simple, but little known. It provides superior efficiency while using commonly available components.

Circuit Description

The circuit is built around a buck converter. Depending on output power, a regulator with integrated fets or a controller with external fets can be used.

The circuit in Figure 1 uses the NCP3170A to obtain a -5 V rail from a +8 V input at up to 0.5A output current. Efficiency reaches 85%

The circuit in Figure 2 converts a +12 V input into -3.3 V or -5V at up to 8A. 90% efficiency is achievable.

There are many more options available for buck controllers and converters as well as for mosfets. The

user needs to pay attention to the voltage rating of the part. In this application the controller and the

mosfets will need to withstand a voltage equal to input voltage plus output voltage. For example the

circuit in figure 1 needs to be able to handle 8 + 5 = 13 V.

Figure 1

Figure 2

Efficiency Curves

50 55 60 65 70 75 80 85 90

0.0 0.1 0.2 0.3 0.4 0.5

Eff (% )

Load (A)

NCP3170A Efficiency

-5V output

80 82 84 86 88 90 92

0 1 2 3 4 5 6 7 8

Eff (% )

Load (A)

NCP3020A Efficiency

-5V output

-3.3V output

Key Features

 Simple and familiar topology

 High efficiency

 Many options available for component selection

References

 Data sheet NCP3170: 3A Switching regulator

 Data sheet NCP3020: 28V Synchronous Buck Controller

1

1 © 2011 ON Semiconductor.

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